A current switch is usually so constructed that emitters of a first transistor and a second transistor are commonly connected to a constant-current supply, and a load resistor is connected between the collector of at least one of the two transistors and the power supply, so that the above-mentioned collector serves as an output terminal. An input voltage is applied to the base of the first transistor, and a reference voltage (or often another input voltage) is applied to the base of the second transistor. With the thus constructed circuit, when the level of the input voltage changes so as to be higher or lower than the level of the reference voltage, one of the transistors is rendered non-conductive and the other one is rendered conductive. However, with the conventional construction of bipolar transistors the collector occupies larger areas than the emitter and establishes a large capacity with respect to the substrate. Thus, the capacity and the load resistor define a time constant which determines the rising speed of the collector potential. Accordingly, the rising speed of the output voltage from the low level to this high level is determined by the time constant. The collector potential is quickly broken from a high level to a low level when the first transistor is rendered conductive according to a small time constant determined by a small resistance while the transistor is conductive and has a capacity Cc. In order to increase the rising speed of the output voltage, it has heretofore been attempted to reduce the load resistance R.sub.1. According to this method, however, increase in the current is not avoidable when it is desired to take out the same logic amplitude as before.